Electrical submersible pumps (ESP) are widely used to pump oil production wells. A typical ESP has a rotary pump driven by an electrical motor. A seal section is located between the pump and the motor to reduce the differential between the well fluid pressure on the exterior of the motor and the lubricant pressure within the motor. A drive shaft, normally in several sections, extends from the motor through the seal section and into the pump for rotating the pump. The pump may be a centrifugal pump having a large number of stages, each stage having an impeller and diffuser.
During operation, the impellers create thrust, which can be downthrust, which acts in an upstream direction and upthrust, which acts in a downstream direction. The impellers transmit the upthrust and downthrust in various manners to the diffusers. If the thrust is severe, wear occurs between the thrust surfaces of the impellers and diffusers. The wear is exacerbated if the well fluid contains abrasive sand particles. Thrust surfaces formed of abrasion resistant material, such as tungsten carbide, may be employed in pumps for use in sand laden wells. Abrasion resistant material thrust surfaces add to the cost of the pump.
The impeller of each stage may have a downstream balance ring that extends into the next downstream diffuser. The downstream balance ring fits closely alongside a wall surface of the downstream diffuser. The close engagement of the downstream balance ring and the diffuser wall surface defines a downstream pressure cavity between the downstream diffuser and the impeller. The close engagement restricts communication of fluid being discharged from the impeller vane passages with fluid in the downstream pressure cavity. The clearance between the downstream balance ring and the diffuser wall surface remains constant whether the impeller is in downthrust or upthrust.
Balance holes may extend through a shroud or downstream side of the impeller, communicating fluid pressure in the vicinity of the intake of the impeller with the downstream pressure cavity. Also, each impeller may have an upstream skirt that is closely spaced from a guide surface of the next upstream diffuser.